What's so great about Joomla 3.0?
There are a range of new features, but (in my view) the headliner is improved mobile support. In the past, we've had to employ various workarounds or use responsive templates to achieve a site that looks good both on mobile devices and on more traditional systems. To a large extent, Joomla! 3.0 takes this burden off our shoulders and does it for us.
As a user, the benefit is simple: In the past, we've had extensions suddenly break. After troubleshooting the reason has been that com_a uses JQuery but so does mod_b. Both have loaded instances of JQuery (without using no_conflict) and the whole thing has fallen apart due to conflicts. With JQuery included by default, there should only ever be one instance loaded - Awesome!
Joomla! 3.0 will also include Twitter Bootstrap.
When Bootstrap was released I was definitely one of the skeptics (who wants every site to look the same?), but having played around with it now see the true potential. Sure, if you just go with the default then every site will look the same, but the true power is that it gives a common starting point for extension and template developers to work from. From a developers perspective, it takes the initial work out of setting up, leading to less effort needed in interface design. For users, that means a better UI which can never be a bad thing! And, of course, Bootstrap is very mobile friendly, so things that broke in the past on mobiles shouldn't cause such an issue now.
These aren't the only new features, but are the main 'newsworthy' ones in my opinion!
Should I Upgrade?
This is a question that the Joomla! team have tried to be very clear on. Joomla! 3.0 is not a Long Term Support (LTS) release, so if you're happy on Joomla! 2.5 then you should probably stay on 2.5 until Joomla! 3.5 is released next year.
The general line of thinking, for many, seems to be that new sites will be implemented using Joomla! 3.0 whilst sites currently on Joomla! 2.5 will stay on 2.5. Of course, there's nothing stopping you upgrading if you want to, it's just that you don't have to.
If you're currently running Joomla! 1.5 then it's more a case of making a choice! Official support for version 1.5 ends very soon, so you should be thinking about upgrading. Whether that's to 2.5 or 3.0 is a personal decision based on your needs, though the advice is generally to opt for the LTS versions (so Joomla! 2.5 and when it's released 3.5)
What about Extensions and Templates
With any major release of software, there can be dramatic changes. Joomla! is no different, and there are some potential backwards compatability issues for extensions (including templates). Most developers will probably be looking to fix these quickly though, and although the list appears incredibly long (I know better than anyone, having typed it all into a script!) the reality is that many of the items listed won't actually affect every extension.
The easiest way to find out whether your extensions work with Joomla 3.0, is either to ask the developer of the extension, or to download a copy of Joomla 3.0 and try them on a test site. Certainly by the time Joomla! 3.5 comes around I'd expect that there'll be a wealth of extensions available, just as there are for 2.5.
Hopefully that has clarified things a little (or at least, not confused things further), though feel free to contact us for more information if it's still not clear (or if I've muddied the waters further).
Bottom line: Joomla 3 is mobile and cool, but 2.5 is still recommended for most users